A halogen lamp that includes a low voltage filament (e.g., rated for operation with a filament voltage of 50 volts or less) provides significant advantages in comparison with a halogen lamp for which the filament is rated for excitation by a typical alternating current (AC) power source (e.g., 120 volts or more). A low voltage halogen lamp operates at a higher current and at a higher temperature, and provides visible light having a color spectrum that is preferred over that which is provided by higher voltage halogen lamps. Moreover, a low voltage halogen lamp has a filament with a larger cross-section area and a shorter length, which makes the lamp last longer. Optically, the filament of a low voltage halogen lamp more closely approximates an ideal point light source, as it provides (in comparison with a filament having a greater length) improved focus with the reflector in the lamp. Thus, a halogen lamp that includes a low voltage filament provides a highly desirable type of illumination.
A low voltage halogen lamp cannot be connected directly to a conventional AC power source. Because the voltage of the AC power source is significantly higher than the rated voltage of the filament/lamp, direct application of the AC power source voltage to the lamp would damage the filament. Consequently, some form of power supply is required in order to at least step down (i.e., reduce) the voltage provided by the AC power source to a level that is suitable for operating the lamp. The predominant power supply for this purpose is commonly referred as an “electronic transformer,” which essentially operates as a constant voltage source. In practice, an electronic transformer is coupled between the AC power source and the halogen lamp, and is generally situated with a housing that is separate from the lamp; with regard to the latter point, safety purposes dictate that the electronic transformer must have an output that is electrically isolated from earth ground in order to preclude any potential hazard involving electrical shock. Typically, such electrical isolation is provided by including an output transformer within the power supply. Unfortunately, an output transformer tends to add significant material cost and physical size, and also detracts from the overall energy efficiency of the power supply.
When the filament of a halogen lamp is “cold” (i.e., as when power is first applied to the lamp), the resistance of the lamp filament is dramatically lower than when the filament is “hot” (i.e., as when power has been applied to the lamp for some time). The resistance of a cold filament can be as low as one-twentieth of the resistance of a hot filament. When the lamp is powered by a circuit (e.g., an electronic transformer) that essentially operates as a constant voltage source, the current that flows through the lamp during an initial period after power is first applied (i.e., when the filament is cold) will be dramatically greater than the steady-state operating current that flows through the lamp once the filament warms up. The relatively high current that flows during the initial period subjects the lamp filament, as well as the components within the power supply, to high amounts of stress that may damage the lamp filament and/or the components within the power supply, and that, over time, negatively impacts the operating life of the lamp and/or the reliability of the power supply.
Therefore, a need exists for a power supply circuit for low voltage halogen lamps that is capable of being realized in a highly economical and energy efficient manner. A need also exists for a power supply circuit that may be readily placed within the base of the lamp. A further need exists for a power supply circuit that reduces and/or limits the lamp current provides to a cold filament so as to protect the filament and the power supply circuit and thereby safeguard the operating life of the lamp and the reliability of the power supply circuit. A power supply circuit with these advantages would represent a considerable advance over the prior art.